<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        .sanjiao {
            border-style: solid;
            border-width: 20px;
            border-color: transparent transparent red transparent;
        }
    </style>
</head>

<body>
    <span class="sanjiao"></span>
</body>

</html>